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We built a free scorecard that shows how your heat pump, ventilation and solar systems are performing. Results are usually surprising.
No thanksWe built a free scorecard that shows how your heat pump, ventilation and solar systems are performing. Results are usually surprising.
No thanks
Positive pressure ventilation systems pull air from your roof cavity, pass it through a filter, and push it down into your living areas through ceiling diffusers. As fresh air enters, slightly stale indoor air is pushed out through natural gaps around doors and windows.
In a New Zealand home, the roof cavity is unlined and unconditioned. On a warm summer day it can reach extreme temperatures. In the middle of winter, particularly overnight or on overcast days, the cavity temperature drops to near the outdoor air temperature.
The system doesn't generate heat. It moves air from where it is to where you are.
In summer, the roof cavity air tends to be warmer than the air inside your home. The system pushes warmer air in. In winter, the opposite is true. The system is still running, still moving air, but the air it's drawing from is cold.
The result is what many homeowners describe: diffusers pushing cool air into bedrooms on winter mornings, the house feeling harder to heat, and power bills that creep up without a clear reason.
The system is operating as designed. It moves air regardless of that air's temperature. Each system has settings designed to reduce the problem. Most homeowners have never been shown these settings exist.
DVS has a Heat Retention setting. When enabled, it automatically reduces the fan speed when the roof air is colder than the air inside your home. DVS also has a Winter mode, which increases airflow automatically when the roof space rises above 19°C on a sunny day, capturing available solar warmth.
The Heat Retention setting ships from the factory switched off. Many DVS systems running in NZ homes right now are operating without it enabled.
Climate Controlled and Reclaim Plus models also have a separate Humidity Sensitivity setting, ranging from Off through Low, Medium, Hi, and V Hi, with Hi as the standard factory setting. This controls how strictly the system holds back roof air that's more humid than the air inside your home, rather than adjusting for indoor humidity directly. A related Humidity Time setting controls how long the system tolerates humid supply air before reacting, from instant response down to a 60-minute delay, with Instant as standard.
On DVS Reclaim Mini units, there's also an indoor humidity limit you or your installer can set. If indoor humidity climbs above that limit, the system automatically boosts to speed level 3 and switches to Through Mode, regardless of the current schedule. This limit isn't fixed at the factory. It's set for your home during installation or service, so it's worth checking what yours is set to rather than assuming a default figure.
SmartVent systems have a minimum temperature protection setting. The factory default is 5°C. When the roof cavity drops below that threshold, the controller turns the fan off or drops it to a very low speed. This setting is adjustable, and 8°C is a more practical threshold for most Auckland homes in winter.
Evolve and Synergy series controllers also have an AUTO Mode. It chooses where to draw air from and what speed to run, based on indoor dew point and temperature, with moisture prioritised over temperature. Rather than snapping to full speed once a threshold is crossed, Evolve gradually scales fan speed up or down to work toward your set humidity and temperature targets. The factory default target on Synergy touchscreens is 75% relative humidity, and it's adjustable in the settings if you want it lower for winter.
Boost, on these systems, is a separate manual button rather than an automatic response. Pressing it runs the fan at maximum speed for 15 minutes, useful if you want a quick burst of extraction rather than waiting for the automatic system to catch up.
HRV's winter mode, what most owners think of as fan speed, is really the ventilation level, a scale from 1 to 6 that controls how much air moves at any given time. Here's how HRV works in winter: the system draws air from the roof cavity regardless of temperature, and the ventilation level determines how much of that cold air comes through versus how much moisture gets cleared.
Higher settings move more air and clear condensation faster, but they also bring more cold air in. Lower settings reduce the cold air but move less moisture out of the house. Adjusting the HRV ventilation level takes under a minute and is the first step before switching the system off or calling a service.
HRV's own guidance suggests levels between 3 and 6 in winter for condensation control. The right starting point depends on your specific home, and getting the calibration right is worth doing properly; see our full HRV control panel guide for the step-by-step method.
Even at low settings, the HRV continues to run at a minimum ventilation level when the roof temperature drops below your set temperature. This keeps enough airflow going to prevent condensation, while causing at most a one-degree change in indoor temperature.
Does HRV automatically adjust for humidity? No. HRV's automation is based on roof and room temperature only, not on measured humidity. Older Classic and Lite controllers show only roof temperature, room temperature, and your set temperature, with no humidity reading at all. The newer AirSense controller does display real-time room humidity on its screen, but it doesn't use that reading to adjust fan speed automatically. On every HRV controller, clearing condensation means manually turning the ventilation level up yourself, ideally to level 4, 5, or 6, when you notice damp windows or mornings. If your controller does show a humidity target, a Humidity Sensitivity setting, or an AUTO Mode that adjusts for moisture, you likely have a DVS or SmartVent system. That's not a genuine HRV unit. The two brands are easy to confuse, since "HRV" gets used generically in NZ for any positive pressure ventilation system.
Not every system in the field has the same controller. Older units, and some of the systems installed in NZ homes are fifteen years old or more, may have simpler controls without automatic temperature thresholds. If you're unsure what your controller can do, your operating manual is the right place to start. DVS, SmartVent and HRV all make manuals available on their websites.
What every system can do, regardless of age or model, is have its fan speed adjusted manually.
If you know a cold night is coming, turning the fan speed down before bed is a practical step. Less speed means less cold air moving through the house overnight. During the day, when the roof cavity tends to warm up, particularly on any sunny winter day, raising the speed again gives the system time to clear moisture and dry the air out.
It isn't a perfect arrangement. But it makes real use of a system that's already installed, and any homeowner can do it without calling anyone.
Lowering your fan speed overnight, whether through your controller settings or manually, may mean waking up to condensation on windows. That's worth knowing before you make the change.
The reason is straightforward. Less airflow overnight means less moisture being pushed out of the house. Cold surfaces, particularly glass, collect that moisture as condensation.
What matters is what happens next. Once the system runs at a higher speed during the day and the roof cavity warms up, that condensation will clear. A house that sees some window condensation in the morning but is well ventilated through the day is in a better position than one where the system has been switched off entirely.
The aim is airflow when it's most useful, during the day, rather than cold air through the night when it's least comfortable and least welcome.
If your system is blowing cold air in winter, check your controller settings before switching it off.
DVS: Look for the Heat Retention setting in your controller menu. If it's switched off, your operating guide has the steps to enable it. Also check the Humidity Sensitivity setting on Climate Controlled and Reclaim Plus models, if fitted.
SmartVent: Check the minimum temperature setting. If cold air is still coming through below the current threshold, raising it to 8°C is a reasonable adjustment for most Auckland homes. Also check the AUTO Mode humidity target on Evolve and Synergy touchscreens, if fitted. For guidance on SmartVent filter replacement, see Smart Vent filter replacement NZ.
HRV: Check your ventilation level. If you're running at 5 or 6 through winter, dropping to 3 or 4 may reduce the cold air without losing meaningful condensation control.
Your operating manual has the exact steps for your model. SmartVent and DVS both have how-to videos on their websites if the manual isn't to hand.If you'd like someone to check the full system, including controller settings and condition, what a professional ventilation service includes covers what that visit should look like. For details on what a positive pressure ventilation service covers, see our positive pressure vesntilation service page.
These systems draw air from your roof cavity and push it into your home through ceiling diffusers. In winter, your roof cavity drops to near outdoor temperatures, particularly overnight or on overcast days. The system moves air regardless of its temperature, so when the cavity is cold, cold air comes in. The fix is not to switch the system off but to check your controller settings. DVS systems have a Heat Retention setting, off by default, that automatically reduces fan speed when roof air is colder than the air inside. SmartVent systems have a minimum temperature protection setting, defaulting to 5°C, which slows or stops the fan below that threshold. Check your operating manual for your specific model.
Switching the system off is not recommended. A ventilation system running at a lower fan speed in winter is more useful than one that has been switched off. Without any airflow, moisture builds inside the house, particularly on windows and cold walls, and mould can establish over a winter. The better approach is to lower the fan speed, either through your controller's winter settings or manually, to reduce the cold air coming in while keeping enough airflow to manage moisture. During the day, when roof cavity temperatures recover, running at a higher speed clears any condensation that built up overnight. Check your controller settings before reaching for the off switch.
HRV recommends ventilation levels between 3 and 6 in winter for condensation control. Higher settings move more cold air into the house. If cold air is your main issue, starting at level 3 is a practical balance. It provides enough airflow to manage moisture without driving cold air through every room at full volume. Adjust from there. If condensation builds on windows overnight, raise the level slightly. If the house feels too cold in the morning, drop it back. The ventilation level is adjusted through the controller. Your operating manual has the steps for your model, and HRV's customer line on 0800 478 123 can walk you through it.
No. HRV controllers, old and new, don't adjust fan speed automatically based on humidity. The system automates based on roof and room temperature only. The newer AirSense controller displays room humidity on screen but doesn't act on it. Managing condensation on an HRV system means manually raising the ventilation level yourself, typically to level 4, 5, or 6 in winter. DVS and SmartVent do have humidity-related automatic features on some models. These work by limiting incoming humid roof air or gradually adjusting fan speed toward a target, not by snapping straight to maximum boost. If your system responds to humidity on its own, it's worth confirming which brand you actually have.
Yes. DVS systems have two winter-related settings. The Heat Retention setting automatically reduces fan speed when the roof air is colder than the air inside your home. The Winter mode increases fan speed automatically when the roof space rises above 19°C on a sunny winter day, capturing free solar warmth. The Heat Retention setting ships from the factory switched off, so many DVS systems in NZ homes are operating without it enabled. Check your controller menu for the Heat Retention setting and refer to your operating guide to enable it. Older DVS models have the same adjustment available through manual fan speed control.
If you have lowered your fan speed or your controller's winter settings have reduced airflow overnight, some morning condensation on windows is normal. Less airflow means less moisture is being pushed out of the house, and cold glass collects that moisture as condensation. This does not mean the system has failed. Once the system runs at a higher speed during the day and the roof cavity warms up, that condensation will clear. A house with some morning window condensation that clears during the day is being managed. If condensation is building and not clearing by midday, your fan speed may be set too low for your home's moisture levels.
HRV draws air from the roof cavity and pushes it into the house through ceiling diffusers, the same as it does year-round. In winter the roof cavity is cold, so the air coming through is cold too. The ventilation level, a scale from 1 to 6, controls how much of that air moves. HRV recommends levels between 3 and 6 in winter to balance condensation control against cold air. The system doesn’t generate heat at any level, it only moves air.
HRV recommends ventilation levels between 3 and 6 in winter for condensation control. The right level depends on your specific home and needs a short calibration period to get right .